To detect the internal humidity of the transformer accurately and sensitively and eliminate the interference caused by electromagnetic fields, an interferometric measurement is proposed in this paper for the first time, to the best of our knowledge. The phase value distribution of the interferogram affected by humidity can be extracted quantitatively. The peak-to-peak value (P-P) of the phase image can reflect the humidity, according to theoretical analysis, and the main factors affecting the P-P are current and humidity. It has been tested by currents of 800 A, 1000 A, and 1200 A and different humidities. The paper reveals the relationship between humidity and P-P, proving that it is a reasonable application of real-time measurement of internal humidity in the transformer.